Systems and methods for controlling a draft inducer for a furnace

ABSTRACT

Systems and methods for controlling a draft inducer for use with a furnace. The draft inducer includes a fan for inducing a draft in the furnace combustion chamber that causes a pressure drop across a heat exchanger assembly for moving exhaust combustion chamber gases. The heat exchanger assembly has a plurality of heat exchanger elements which determine furnace capacity wherein the pressure drop across the heat exchanger assembly is affected by the furnace capacity. A motor drives the fan in response to a motor control signal and a memory stores information defining a relationship between motor speed, motor torque and parameters defining motor operation. A processor determines the speed and torque of the motor and retrieves from the memory the parameters corresponding to the determined motor speed and motor torque. The processor is responsive to the retrieved parameters for defining at least one set of speed/torque curves corresponding to a desired pressure drop across the heat exchanger assembly. A control circuit generates the motor control signal in response to the defined set of speed/torque curves to cause the motor to operate as a function of the determined motor speed and motor torque which are indicative of the furnace capacity for controlling the draft induced in the combustion chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of 1) commonly assignedapplication Ser. No. 08/025,371 filed Feb. 26, 1993 now U.S. Pat. No.5,418,438; 2) commonly assigned application Ser. No. 08/299,528 filedSep. 1, 1994 now U.S. Pat. No. 5,557,182 which application Ser. No.08/299,528 is a continuation-in-part of application Ser. No. 08/025,371;3) commonly assigned application Ser. No. 08/397,686 filed Mar. 1, 1995,now abandoned which application Ser. No. 08/397,686 is acontinuation-in-part of application Ser. No. 08/025,371, applicationSer. No. 08/299,528, and commonly assigned application Ser. No.08/352,393 filed Dec. 8, 1994 (pending) which application Ser. No.08/352,393 is a continuation of commonly assigned application Ser. No.08/023,790 filed Feb. 22, 1993 (abandoned); and 4) commonly assignedapplication Ser. No. 08/402,998 filed Mar. 9, 1995 (pending) whichapplication Ser. No. 08/402,998 is a continuation-in-part of applicationSer. No. 08/025,371, application Ser. No. 08/299,528, application Ser.No. 08/352,393, and application Ser. No. 08/397,686; the entiredisclosures of which are incorporated herein by reference.

NOTICE

Copyright ©1995 General Electric Company. A portion of the disclosure ofthis patent document contains material which is subject to copyrightprotection. The copyright owner has no objection to the facsimilereproduction by anyone of the patent document or the patent disclosure,as it appears in the Patent and Trademark Office patent files orrecords, but otherwise reserves all copyright rights whatsoever.

BACKGROUND OF THE INVENTION

The invention generally relates to electronically controlled motors and,in particular, to a draft inducer system providing improved control fora motor driving a fan for inducing a draft in an exhaust.

In a conventional furnace, natural convection currents move air throughthe exhaust outlet of the furnace's combustion chamber to carry awayexhaust gases. The moving air further induces a draft in the combustionchamber for mixing oxygen with the fuel being burned in the chamber.Heat energy remaining in the exhaust gases, however, is lost to theatmosphere which decreases the overall efficiency of the furnace. Theuse of heat exchangers improves furnace efficiency by extractingadditional heat from the exhaust gases before they are vented to theatmosphere. Extracting heat from the exhaust gases, however, reduces thenatural convection currents which would otherwise carry the gases away.One solution has been to use a draft inducing fan to force the exhaustgases into the atmosphere.

Commonly assigned U.S. Pat. No. 5,075,608 and application Ser. No.08/025,371, the entire disclosures of which are incorporated herein byreference, provide improvements in draft inducer systems whichbeneficially increase furnace efficiency. Such improvements allowcontrolling fan speed as a function of the pressure sensed by a pressuretransducer. Further improvements provided by application Ser. No.08/299,528, the entire disclosure of which is incorporated herein byreference, minimize the risk of overheating the electronic control of adraft inducer system by maintaining motor operation within a safe area.

Application Ser. No. 08/402,998 and 08/397,686, the entire disclosuresof which are incorporated herein by reference, provide furtherimprovements in draft inducer systems for maintaining pressure acrossthe furnace's heat exchanger assembly at a desired level without the useof a pressure transducer for controlling motor speed. These applicationsfurther provide improvements in sensing ignition in the combustionchamber since combustion decreases the density of the combustion chambergases moved by the draft inducer fan which can affect motorspeed/torque.

While such systems represent improvements, further improvements in draftinducer control systems, draft inducer apparatus and methods of controland operation are needed to economically and efficiently accommodatefurnaces having different restrictions to air flow which affects draftinducer performance. Generally, furnaces with different capacities willhave different restrictions and, thus, will require the draft inducermotor to operate at different speeds and/or torques to produce thedesired pressure in the combustion chamber. For this reason,conventional furnaces either use different motors for differentcapacities or include a device, such as a choke plate, to modify thefurnace installation. A choke plate restricts air flow to "simulate" ahigher capacity furnace. However, the use of different motors or themodification of the furnace increases both cost and the complexity ofinstallation. Therefore, improvements are needed to beneficially providemotor control which is adaptable to furnaces of differing capacities.

SUMMARY OF THE INVENTION

Among the objects of the invention are to provide an improved motorsystem and method of control which overcome at least some of thedisadvantageous conditions described above; to provide such a motorsystem and method which permit adaptively controlling motor operationbased on the furnace restriction; to provide such a motor system andmethod which permit determining furnace capacity as a function offurnace restriction; to provide such a motor system and method whichpermit adaptively controlling motor operation based on the furnacecapacity; to provide such a motor system and method which permitmaintaining a desired pressure drop across a heat exchanger assembly forextracting heat from the combustion chamber's exhaust; to provide such amotor system and method which permit maintaining a desiredair/combustion material mixture; and to provide such a motor system andmethod which are electrically efficient, reliable, economical andconvenient to use.

Briefly described, a draft inducer apparatus embodying aspects of thepresent invention is for use with a furnace that has a combustionchamber and an exhaust outlet for venting exhaust combustion chambergases from the furnace. The furnace also has at least one heat exchangerassembly for extracting heat from the exhaust combustion chamber gasesand transferring heat to conditioned air. The apparatus is also for usewith a fan for inducing a draft in the combustion chamber which causes apressure drop across the heat exchanger assembly for moving the exhaustcombustion chamber gases through the exhaust outlet. The heat exchangerassembly has a plurality of heat exchanger elements which determinefurnace capacity wherein the pressure drop across the heat exchangerassembly is affected by the furnace capacity. The apparatus includes amotor with a shaft for driving the fan in response to a motor controlsignal and a memory storing information defining a relationship betweenmotor speed, motor torque and parameters defining motor operation.According to the invention, a processor determines the speed and torqueof the motor and retrieves from the memory a parameter corresponding tothe determined motor speed and motor torque. The processor is responsiveto the retrieved parameter for defining at least one set of speed/torquecurves corresponding to a desired pressure drop across the heatexchanger assembly. The apparatus further includes a control circuit forgenerating the motor control signal in response to the defined set ofspeed/torque curves. Thus, the motor will operate as a function of thedetermined motor speed and motor torque, which are indicative of thefurnace capacity, to control the draft induced in the combustionchamber.

In another form, a furnace system embodying aspects of the presentinvention includes a Combustion chamber and an exhaust outlet forventing exhaust combustion chamber gases from the furnace system. Thefurnace system also includes at

least one heat exchanger assembly for extracting heat from the exhaustcombustion chamber gases and transferring heat to conditioned air. Thefurnace system further has a fan for inducing a draft in the combustionchamber which causes a pressure drop across the heat exchanger assemblyfor moving the exhaust combustion chamber gases through the exhaustoutlet. The heat exchanger assembly has a plurality of heat exchangerelements which determine furnace capacity wherein the pressure dropacross the heat exchanger assembly is affected by the furnace capacity.The furnace system further includes a motor with a shaft for driving thefan in response to a motor control signal and a memory storinginformation defining a relationship between motor speed, motor torqueand parameters defining motor operation. According to the invention, aprocessor determines the speed and torque of the motor and retrievesfrom the memory a parameter corresponding to the determined motor speedand motor torque. The processor is responsive to the retrieved parameterfor defining at least one set of speed/torque curves corresponding to adesired pressure drop across the heat exchanger assembly. The furnacesystem further includes a control circuit for generating the motorcontrol signal in response to the defined set of speed/torque curves.Thus, the motor will operate as a function of the determined motor speedand motor torque, which are indicative of the furnace capacity, tocontrol the draft induced in the combustion chamber.

Generally, another form of the invention is a method of operating adraft inducer apparatus for use with a furnace that has a combustionchamber and an exhaust outlet for venting exhaust combustion chambergases from the furnace. The furnace also has at least one heat exchangerassembly for extracting heat from the exhaust combustion chamber gasesand transferring heat to conditioned air. The draft inducer apparatus isalso for use with a fan for inducing a draft in the combustion chamberwhich causes a pressure drop across the heat exchanger assembly formoving the exhaust combustion chamber gases through the exhaust outlet.The heat exchanger assembly has a plurality of heat exchanger elementswhich determine furnace capacity wherein the pressure drop across theheat exchanger assembly is affected by the furnace capacity. The methodincludes the steps of driving the fan with a motor in response to amotor control signal and storing information in a memory defining arelationship between motor speed, motor torque and parameters definingmotor operation. The method also includes determining the speed andtorque of the motor and retrieving from the memory a parametercorresponding to the determined motor speed and motor torque. Accordingto the invention, the method includes the steps of defining at least oneset of speed/torque curves corresponding to a desired pressure dropacross the heat exchanger assembly in response to the retrievedparameter and generating the motor control signal in response to thedefined set of speed/torque curves. Thus, the motor will operate as afunction of the determined motor speed and motor torque, which areindicative of the furnace capacity, to control the draft induced in thecombustion chamber.

Other objects and features will be in part apparent and in part pointedout hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a furnace system including a controlcircuit for a motor according to a preferred embodiment of theinvention.

FIG. 1A is a block diagram of a furnace system including a controlcircuit for a motor according to another preferred embodiment of theinvention.

FIG. 2 illustrates exemplary speed vs. torque curves defining fouroperating states of the system of FIG. 1.

FIGS. 3A, 3B and 3C illustrate a flow diagram of the operation of thecontrol circuit of FIG. 1 according to a preferred embodiment of theinvention.

FIG. 4 is a perspective view of a furnace according to a preferredembodiment of the invention with portions cut away.

FIGS. 5A and 5B illustrate a flow diagram of the operation of thecontrol circuit of FIG. 1 according to a preferred embodiment of theinvention.

FIGS. 6A and 6B illustrate a flow diagram of the operation of thecontrol circuit of FIG. 1 according to another preferred embodiment ofthe invention.

FIGS. 7A and 7B illustrate a flow diagram of the operation of thecontrol circuit of FIG. 1 according to yet another preferred embodimentof the invention.

FIGS. 8A and 8B illustrate a flow diagram of the operation of thecontrol circuit of FIG. 1 according to yet another preferred embodimentof the invention.

FIGS. 9A, 9B, 9C, 9D, 9E and 9F illustrate exemplary speed vs. torquecurves defining operation of the motor of FIG. 1.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, one preferred embodiment of a system 100 isshown, including a control circuit 102 for any electronicallycontrollable motor. Such motors include single and variable speedmotors, selectable speed motors having a plurality of finite, discretespeeds and brushless DC motors, including electronically commutatedmotors and switched reluctance motors. In the illustrated embodiment,the control circuit 102 is connected to a motor 104. The motor 104 ispreferably a draft inducer variable speed motor such as the single phasemotor described in commonly assigned application Ser. No. 08/352,393,the entire disclosure of which is incorporated herein by reference.Control circuit 102 is preferably mounted within a housing (not shown)of motor 104 and sends control commands to motor 104 via line 106 to aset of power switches 108. Control circuit 102 also receives speed ortorque feedback from motor 104 via line 110. In a preferred embodiment,motor 104 has a stationary assembly (not shown) and a rotatable assembly(not shown) in magnetic coupling relation to the stationary assembly.The stationary assembly includes windings adapted to be energized in atleast one preselected sequence. The power switches 108, responsive tothe control commands at line 106, selectively connect a power supply 112to the windings of motor 104 to provide current to the windings in thepreselected sequence to produce an electromagnetic field for rotatingthe rotatable assembly. The rotatable assembly of motor 104 is coupledvia a shaft 113 to a rotatable component, such as a fan 114, forinducing a draft in an exhaust outlet 116 of a conventional heating,ventilating, and air conditioning system, such as a furnace 117(portions of which are shown in FIGS. 1 and 4).

According to the invention, the system 100 is for use with the furnace117 and, in the alternative, system 100 includes furnace 117.Preferably, furnace 117 has a combustion chamber 118 providingcombustion gases to which the exhaust outlet 116 is connected forventing exhaust combustion gases from furnace 117. Fan 114, which ispositioned within exhaust outlet 116 in one embodiment, induces a draftin the combustion chamber 118 by moving exhaust combustion chamber gasesthrough exhaust outlet 116. As a result of the moving gases, a pressureis established in exhaust outlet 116. In an alternative embodiment, fan114 is positioned in an inlet (not shown) to combustion chamber 118.

Furnace 117 conditions air for heating a space (not shown) by moving airwith a blower (see FIG. 4) across a heat exchanger assembly 120positioned adjacent combustion chamber 118. In this manner, theconditioned air gains heat before it is discharged to the space by theblower. As described above, natural convection forces cause hot exhaustgases to rise and vent to the atmosphere in a typical furnace. In apreferred embodiment, the heat exchanger assembly 120 also extracts heatfrom the exhaust combustion chamber gases before they are vented away.Additional pressure, however, is then needed to force the cooled exhaustthrough exhaust outlet 116 to the atmosphere via an exhaust port 122.Inducing a draft with fan 114 provides the additional pressure. In thismanner, fan 114 develops a positive flue pressure which forces furnaceexhaust from combustion chamber 118 through heat exchanger assembly 120(where heat is extracted and transferred to conditioned air provided tothe space to be heated) and then forces cooled exhaust to vent via theexhaust port 122 of exhaust outlet 116. As described herein, exhaustcombustion chamber gases include products of combustion, fuel gasesand/or input air. Thus, when combustion chamber 118 is not ignited, theexhaust combustion chamber gases primarily constitute air being movedthrough combustion chamber 118 by fan 114.

In another preferred embodiment, shown in FIG. 1A, system 100 includesan additional heat exchanger assembly 120a for extracting heat from theexhaust combustion chamber gases. It is to be understood that thepositional relationships between exhaust outlet 116, combustion chamber118, heat exchanger assembly 120 and/or heat exchanger assembly 120a mayvary depending on the particular furnace. Further, the number of heatexchanger assemblies or the number of heat exchanger elements used inthe particular furnace may vary as well. In a preferred embodiment, afurnace according to the present invention is of a general type, such asa two-stage condensing furnace. However, a furnace according to theinvention could be a single or multiple stage, or variable stage,condensing or non-condensing furnace. For clarity, the present inventionwill be described with respect to heat exchanger assembly 120 of FIG. 1where corresponding reference characters throughout FIGS. 1 and 1Aindicate corresponding parts.

According to the invention, furnace 117 preferably operates in at leasttwo operating states. In a first, or pre-fire, operating state,combustion does not occur in combustion chamber 118. In a second, orpost-fire, operating state, however, a fuel supply 124 provides acombustible material (i.e., a fuel such as natural gas or oil) tocombustion chamber 118. An igniter 126, activated by a furnacecontroller 128 via line 130, ignites the combustible fuel within chamber118. In the second operating state, combustion occurs in combustionchamber 118. In another preferred embodiment of the invention, thefurnace controller 128 is part of control circuit 102 which providescontrol not only for motor 104 but also for furnace 117.

As a general rule, the density of the combustion chamber gases flowingthrough or across heat exchanger assembly 120 and fan 114 is greater inthe first operating state, without combustion, than in the secondoperating state, with combustion. As a result, the speed of fan 114increases when furnace 117 changes from the first operating state to thesecond operating state. Similarly, in a system operating such that speedremains constant, motor torque will decrease as the density of thecombustion chamber gases flowing across fan 114 decreases.

The speed of fan 114 as driven by motor 104 regulates the flow ofcombustion chamber gases in system 100. In systems where combustionby-products need to be controlled, it is important to achieve the properamount of input air mixed with fuel so that an optimum air-fuel mixtureis constantly being burned in combustion chamber 118. To achieve theproper air-fuel mixture, combustion products must be exhausted at anappropriate rate.

As described above, the density of combustion chamber gases flowingacross heat exchanger assembly 120 is greater when furnace 117 isoperating in its first operating state rather than in its secondoperating state. According to the invention, the control commands online 106 take the form of a motor control signal. Motor 104 drives fan114 in response to an appropriate motor control signal at a motor speedwhich is a function of the density of the combustion chamber gasesflowing across heat exchanger assembly 120. As such, different motorspeeds result as a function of the density of the combustion chambergases flowing across fan 114. In turn, fan 114 moves the combustionchamber gases through exhaust outlet 116 thereby inducing a draft incombustion chamber 118 that causes a pressure drop across heat exchangerassembly 120.

A speed circuit, preferably resident in control circuit 102, provides aspeed signal representative of the speed of motor 104 in response to thespeed/torque feedback via line 110. In the alternative, the speed signalcould be provided by a speed sensor external to the housing of motor104. U.S. Pat. No. 5,075,608 discloses a draft inducer control systemwhich controls fan speed in response to the pressure measured by apressure transducer located between the fan and combustion chamber.Advantageously, system 100 accomplishes speed regulation to maintain adesired pressure across heat exchanger assembly 120 as a function ofpredefined speed/torque curves rather than by using a pressure sensor.Thus, the need for a pressure sensor for regulating motor speed iseliminated.

Control circuit 102 generates motor control signals in response to thespeed signal as a function of either a first set of speed/torque curvesor a second set of speed/torque curves. Prior to operating system 100,the speed/torque curves are developed by running a sample furnace undervarying conditions. For example, a motor similar to motor 104 drives afan similar to fan 114 for moving combustion chamber gases at a givendensity and at a given fuel flow to the sample furnace's combustionchamber. An operator varies the duty cycle of current energizing themotor's windings until a desired pressure drop across heat exchangerassembly 120 is reached in the sample furnace and then measures motorspeed. By varying the restrictions to air flow in the sample furnace, anumber of samples can be taken for developing speed/torque curves. Forexample, a speed/torque curve is developed by performing a three-piecelinear fit of four samples taken at various air restrictions. As aspecific example, for a pressure of 1.8 inches, a current value of 90(where 0 corresponds to 0% duty cycle and 255 corresponds to 100% dutycycle) yields a minimum speed of 2100 at a minimum air restriction; anda current value of 160 yields a maximum speed of 4300 rpm at a maximumacceptable air restriction. Two intermediate points are obtained with acurrent value of 110 and a speed of 2700 rpm and with a current value of130 and a speed of 3400 rpm. The duty cycle is a convenient means forgenerating speed/torque curves during testing of motor 104. Further, bychanging the air restrictions, the operator simulates various lengths ofpiping connected to furnace 117. Also, the maximum air restriction isselected as a function of noise, the maximum motor speed and the like.The process is then repeated for low density combustion chamber gases,that is, with combustion in the sample furnace's combustion chamber.Similarly, speed/torque curves are developed for different fuel flowsrequiring different pressures, that is, low and high stage furnaceoperation. The operator can further modify the curves by varying theadvance angles and off times of the current energizing the motor'swindings to achieve desired performance.

In one preferred embodiment, control circuit 102 includes a memory 132for storing the first and second sets of speed/torque curves as a tableof predefined speed/torque values corresponding to desired operation offurnace 117 under varying conditions, including various restrictions toair flow. Thus, the memory 132 defines the first and second sets ofspeed/torque curves corresponding to a desired pressure drop across heatexchanger assembly 120.

As described above, control circuit 102 generates motor control signalsin response to the speed signal as a function of either the first orsecond set of speed/torque curves. According to a preferred embodimentof the invention, control circuit 102 generates the motor control signalas a function of the first set of speed/torque curves until the speedsignal indicates that the speed of motor 104 has reached a predeterminedspeed. After the speed signal indicates that the speed of motor 104 hasreached the predetermined speed, control circuit 102 generates the motorcontrol signal as a function of the second set of speed/torque curves.In this manner, motor 104 will operate in accordance with one or more ofthe first speed/torque curves when furnace 117 is in the first operatingstate and in accordance with one or more of the second speed/torquecurves when furnace 117 is in the second operating state.

In addition to storing the first and second sets of speed/torque curves,memory 132 preferably stores the speed of motor 104 as represented bythe speed signal a predetermined period of time after motor 104 firstbegins driving fan 114. A circuit resident in control circuit 102compares the stored speed to the motor speed as subsequently representedby the speed signal. In this manner, control circuit 102 detects achange from the first operating state to the second operating state whenthe difference between the compared speeds exceeds a predeterminedamount. A change in the density of the combustion chamber gases flowingacross heat exchanger assembly 120 causes this speed difference. Assuch, control circuit 102 provides adaptive control.

The furnace system of the invention includes furnace controller 128which provides a furnace operating signal for causing furnace 117 tooperate in either a low stage or a high stage of the first and secondoperating states. The low stage corresponds to a low desired flow offuel to combustion chamber 118 and to a low desired pressure drop acrossheat exchanger assembly 120. Likewise, the high stage corresponds to ahigh desired flow of fuel to combustion chamber 118 greater than the lowdesired flow and to a high desired pressure drop across heat exchangerassembly 120 greater than the low desired pressure drop. As an example,the low delta pressure may be 1.05 inches and the high delta pressuremay be 2.2 inches for heat exchanger assembly 120. Furnace controller128 communicates the desired low or high stage to the fuel supply 124via line 134 for varying the amount of fuel supplied to combustionchamber 118.

The first and second sets of speed/torque curves each include aspeed/torque curve corresponding to the low stage and a speed/torquecurve corresponding to the high stage. Thus, in a preferred embodimentof the invention, memory 132 stores a pre-fire low stage speed/torquecurve and a pre-fire high stage speed/torque curve in addition to apost-fire low stage speed/torque curve and a post-fire high stagespeed/torque curve. Control circuit 102 generates the appropriate motorcontrol signal in response to the furnace operating signal as a functionof the pre-fire low stage speed/torque curve in the low stage of thefirst operating state and as a function of the pre-fire high stagespeed/torque curve in the high stage of the first operating state.Likewise, control circuit 102 further generates the motor control signalas a function of the post-fire low stage speed/torque curve in the lowstage of the second operating state and as a function of the post-firehigh stage speed/torque curve in the high stage of the second operatingstate.

Furnace controller 128 also communicates the desired low or high stageto control circuit 102 via line 136. When changing in-state from the lowstage to the high stage, control circuit 102 generates a motor controlsignal independent of the speed/torque curves. For example, if furnace117 is operating in the low stage of the second operating state, theindependent motor control signal adds a current value of 80 to thepresent low stage current value for a period of time, such as 1.5seconds. This causes motor 104 to rapidly increase its speed beforecontrol circuit 102 generates the motor control signal as a function ofthe post-fire high stage speed/torque curve. Similarly, when changingin-state from the high stage to the low stage, control circuit 102 alsogenerates a motor control signal independent of the speed/torque curves.For example, if furnace 117 is operating in the high stage of the secondoperating state, the independent motor control signal subtracts acurrent value of 40 from the present high stage current value for aperiod of time, such as 1.5 seconds. This causes motor 104 to rapidlydecrease its speed before control circuit 102 generates the motorcontrol signal as a function of the post-fire low stage speed/torquecurve.

According to the invention, furnace 117 discharges heated conditionedair to the space in response to a thermostatic control 138. Thethermostatic control 138 preferably provides a two state thermostatsignal via line 140 to furnace controller 128 as a function of thetemperature of the air in the space. Furnace controller 128 beginsoperation of furnace 117 in the first operating state in response to aDEMAND state of the thermostat signal. Conversely, furnace controller128 ends operation of furnace 117 in the second operating state inresponse to a NO DEMAND state of the thermostat signal. Furnacecontroller 128 communicates the state of thermostatic control 138 vialine 142. In response to the NO DEMAND state, control circuit 102generates a motor control signal for causing motor 104 to drive fan 114for a predetermined period of time, such as 15 seconds, after furnace117 ends operation in the second operating state. In this manner, fan114 clears combustion chamber gases, including unburned fuel andremaining exhaust, from exhaust outlet 116. It is to be understood thatthe DEMAND state of the thermostat control signal can correspond to thelow stage or high stage or an intermediate stage depending on theparticular operating parameters of furnace 117 and the temperature inthe space.

In a preferred embodiment, a reset circuit 144, alternatively embodiedresident in control circuit 102, is responsive to the NO DEMAND state toreset control circuit 102 after furnace 117 ends operation in the secondoperating state.

Furnace 117 further includes a fuel control resident in furnacecontroller 128 for providing a fuel signal which represents whether fuelsupply 124 is supplying fuel to combustion chamber 118. In analternative embodiment, memory 132 is responsive to the fuel signal vialine 136 for storing the speed of motor 104 as represented by the speedsignal when the fuel signal indicates that fuel supply 124 is notsupplying fuel to combustion chamber 118. Control circuit 102 thencompares the stored speed to the motor speed when the fuel signalindicates that fuel is being supplied to combustion chamber 118.

In another alternative embodiment of the invention, system 100 includesa pressure switch 146, shown in phantom in FIG. 1. The pressure switch146, preferably located within exhaust outlet 116 between fan 114 andcombustion chamber 118, functions as a safety backup for system 100. Asan example, if the pressure in exhaust outlet 116 falls below aparticular level, the risk of a backdraft or a buildup of exhaust gasesincreases. Pressure switch 146 preferably detects when the pressure dropacross heat exchanger assembly 120 has fallen below an acceptableminimum pressure. If so, furnace 117 is instructed via line 148 to shutoff the flow of fuel to combustion chamber 118. Thus, pressure switch146 functions as a safety backup feature by disabling furnace 117 whenthe pressure drop across heat exchanger assembly 120 becomes low. In yetanother alternative embodiment, pressure switch 146 communicates apressure signal representative of the minimum pressure across heatexchanger assembly 120 directly to control circuit 102.

FIG. 2 illustrates exemplary speed vs. torque curves for motor 104driving draft inducer fan 114. Control circuit 102 preferably controlspowers switches 108 in accordance with the motor control signal whichrepresents a desired current signal based on the speed/torque curves. Ineffect, the desired current signal corresponds to a desired torquesignal because the torque of the motor is related to the motor currentas shown in FIG. 2. The control commands at line 106 are a function ofthe desired current signal and cause power switches 108 to selectivelyenergize the windings of motor 104.

In a preferred embodiment, the desired current signal is a mapping ofcurrent values corresponding to a duty cycle input to motor 104. Theduty cycle is a convenient means for generating speed/torque curvesduring testing of motor 104. FIG. 2 illustrates the speed/torque curvesassociated with duty cycles of 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%and 100%.

In this embodiment of the invention, each curve is representative of thetorque produced by motor 104 when its windings are energized by asubstantially constant current with respect to speed. As shown by thesecurves, the windings of motor 104 in this embodiment of the inventionhave a relatively high inductance and less torque is produced inresponse to a particular current demand at high speeds than at lowspeeds. For clarity, the speed/torque curves of FIG. 2 are illustratedas smooth lines. However, the actual speed/torque response illustratedby these curves include variations caused by switching noise and thelike. Further, it is to be understood that the curves may vary forapplications other than those specifically disclosed herein.

According to the invention, operation of motor 104 is controlledaccording to one of first speed/torque operating curves 202 and 204 orone of second speed/torque operating curves 206 and 208. As shown, thecurve 202 corresponds to pre-fire low stage operation and the curve 204corresponds to pre-fire high stage operation. Likewise, the curve 206corresponds to post-fire low stage operation and the curve 208corresponds to post-fire high stage operation. Generally, each of curves202, 204, 206 and 208 represent operation resulting in a relativelyconstant desired pressure drop across heat exchanger assembly 120.However, it is to be understood that for certain applications of theinvention, the desired pressure may be, for example, slightly greater atlower motor speeds. Further, each of curves 202, 204, 206 and 208 aremerely exemplary and are to be used for a particular furnace.

In operation of system 100, thermostatic control 138 provides the DEMANDsignal requesting heating in the space. In response, furnace controller128 selects either high stage or low stage operation. Control circuit102 generates the appropriate motor control signals for beginningoperation of motor 104 according to either curve 202 or 204 asdetermined by furnace controller 128. For clarity, low stage operationwill be described first.

Control circuit 102 generates the motor control signal to beginoperation of motor 104 at the minimum desired speed for pre-fire lowstage operation, as shown at point 210 on curve 202. Control circuit 102operates according to an algorithm embodied in flow diagram form inFIGS. 3A-3C. Motor 104 seeks a steady state speed on curve 202 accordingto the air restriction in furnace 117 of system 100 between point 210and the maximum desired speed for pre-fire low stage operation, as shownat point 212 on curve 202. After a predetermined period of time, such as21 seconds, the steady state speed of motor 104, as represented by thespeed signal, is stored in memory 132 of control circuit 102. After 40seconds, for example, system 100 continuously monitors motor speed.

Furnace controller 128 then commands fuel supply 124 via line 134 tosupply combustion chamber 118 with fuel according to the desired lowstage fuel flow. By operating motor 104 according to curve 202, system100 maintains the pressure drop across heat exchanger assembly 120 at adesired level and, thus, provides a desired air-fuel mixture incombustion chamber 118. Furnace controller 128 also activates igniter126 via line 130 so that the air-fuel mixture burns. As the combustionchamber gases flowing across heat exchanger assembly 120 heat up, thedensity decreases resulting in a relatively sudden increase in the speedof fan 114. By monitoring the speed of motor 104 and comparing thepresent speed to the speed stored in memory 132, control circuit 102detects fire in combustion chamber 118 when the present speed exceedsthe stored speed by a predetermined amount, such as 275 rpm. As a resultof the sudden increase in speed, the pressure across heat exchangerassembly 120 also increases momentarily until control circuit 102switches its operation to post-fire low stage curve 206. Motor 104 thenseeks a steady state speed on curve 206 according to the air restrictionin furnace 117 of system 100 between the minimum desired speed forpost-fire low stage operation, as shown at point 214 on curve 206, andthe maximum desired speed for post-fire low stage operation, as shown atpoint 216 on curve 206.

As shown in FIG. 2, points 218 and 220 indicate the minimum and maximum,respectively, desired speeds for pre-fire high stage operation offurnace 117. Likewise, points 222 and 224 indicate the minimum andmaximum, respectively, desired speeds for post-fire high stage operationof furnace 117.

FIGS. 3A, 3B and 3C illustrate the operation of system 100 in the formof a flow diagram 300. In a preferred embodiment of the invention,control circuit 102 includes a processor 150 (see FIG. 1) for performingthe operations illustrated by the flow diagram 300.

After beginning at step 302, control circuit 102 determines at step 304whether motor 104 has changed state since the last time the processor150 performed the steps of flow diagram 300. In this instance, the term"state" refers to a commutation of the windings of motor 104. For asingle phase motor, a "state" corresponds to 180 electrical degrees. Ifmotor 104 has not changed state, then flow diagram 300 ends. If,however, motor 104 has changed state, control circuit 102 proceeds tostep 306. At step 306, control circuit 102 determines whether the modeof furnace 117 has changed within a period to time, n. In other words,control circuit 102 determines whether furnace 117 changed from the lowstage to the high stage, or vice versa, of the second operating stateduring the previous n seconds. As an example, n=1.5 seconds. If the modeof furnace 117 has changed, flow diagram 300 ends. On the other hand, ifthe furnace mode has not changed, control circuit 102 calculates thespeed of motor 104 at step 308 as a function of the time it takes forthe rotatable assembly of motor 104 to complete a revolution. This isaccomplished by dividing a constant k₁ by the time motor 104 spent inthe commutation state.

Control circuit 102 then proceeds to step 310 for comparing the timemotor 104 has been operating in the particular furnace mode (high stageor low stage) to a predetermined period of time, TOFSAV. The time periodTOFSAV is preferably set for a particular furnace controller 128 andcorresponds to the time necessary for motor 104 to reach a relativelysteady state speed. As an example, TOFSAV=21 seconds. Thus, memory 124stores the speed of motor 104 as calculated in step 308 and asrepresented by the speed signal as SPEEDSAV at step 312 when the time inthe operating state equals TOFSAV. Thereafter, control circuit 102monitors the speed of motor 102 when the time in the operating stateexceeds another predetermined time period, m, at step 314. As anexample, m=40 seconds for a particular furnace which typically ignitesafter 45 seconds. In this manner, the risk of false indications ofoperating state changes between 21 and 40 seconds is eliminated.

After the time in the operating state exceeds m, control circuit 102proceeds to step 316 for determining whether the present speed of motor104 exceeds the stored speed by a predetermined amount. Particularly,the speed of motor 104 is compared to (SPEEDSAV+k₂). As an example, k₂=275 rpm. If motor speed exceeds (SPEEDSAV+k₂), then control circuit 102determines that the density of the combustion chamber gases flowingacross heat exchanger assembly 120 has decreased by an amount indicativeof a change in operating state of furnace 117. Thus, control circuit 102sets a FIREFLAG=1 at step 318. In setting FIREFLAG=1, control circuit102 is indicating that furnace 117 changed from the first, or pre-fire,operating state to the second, or post-fire, operating state.

In an alternative embodiment, control circuit 102 compares the speed ofmotor 104 to (SPEEDSAV2+k₂) where k₂ is a negative number, such as -275rpm, and SPEEDSAV2 is the motor speed saved at a time period aftercombustion is sensed. If motor speed is less than (SPEEDSAV2+k₂), thencontrol circuit 102 determines that the density of the combustionchamber gases flowing across heat exchanger assembly 120 has increasedby an amount indicative of a change in operating state of furnace 117from the second operating state to the first operating state. In otherwords, control circuit 102 alternatively detects a loss of flame incombustion chamber 118 by the relatively sudden decrease in motor speed.

Referring again to flow diagram 300, control circuit 102 proceeds tostep 320 if the time in the operating state is less than m. The nextsteps 320, 322, 324 and 326 represent particular safety features ofcontrol circuit 102 for signaling furnace controller 128 that the flowof combustion chamber gases is outside the design limits of furnace 117.In step 320, control circuit 102 compares the speed of motor 104 to aminimum speed. In a preferred embodiment, the minimum speed isdetermined by the speed/torque curves 202, 204, 206 and 208 of FIG. 2depending on the particular operating state and mode (high or low stage)of furnace 117. If motor speed is less than the minimum (see points 210,214, 218 and 222 shown in FIG. 2), control circuit 102 disables the rpmoutput at step 322. In one preferred embodiment, control circuit 102provides an alternating current signal having its frequency proportionalto the motor speed. As disclosed in application Ser. No. 08/025,371,furnace controller 128 receives the ac signal via an interface (notshown). Thus, disabling the rpm output instructs furnace controller 128to shut off fuel flow to combustion chamber 118 because furnace 117 isnot in an acceptable operating pressure range.

Similarly, if the speed of motor 104 is not less than the minimum,control circuit 102 proceeds to step 324. In step 324, control circuit102 compares the speed of motor 104 to a maximum speed. Again, themaximum speed is determined by the speed/torque curves 202, 204, 206 and208 of FIG. 2 depending on the particular operating state and mode (highor low stage) of furnace 117. If motor speed exceeds the maximum (seepoints 212, 216, 220 and 224 shown in FIG. 2), control circuit 102disables the rpm output at step 326. However, if motor speed is lessthan the maximum and greater than the minimum, control circuit 102proceeds to step 328.

At step 308, control circuit 102 has already determined that motor 104is operating within an acceptable speed range. Thus, control circuit 102performs the three-piece linear fit at step 328 using one ofspeed/torque curves stored in memory 132, such as one of curves 202, 204or 206, 208. In other words, processor 150 of control circuit 102 inputsmotor speed and calculates the desired current according to theappropriate speed/torque curve. At step 330, control circuit 102generates the motor control signal to provide the desired current to thewindings of motor 104. In this manner, motor 104 will operate inaccordance with one of the first speed/torque curves when furnace 117 isin the first operating state and in accordance with one of the secondspeed/torque curves when furnace 117 is in the second operating state.Flow diagram 300 ends at step 332.

Referring now to FIG. 4, one preferred embodiment of system 100 is foruse with furnace 117, portions of which are described above, includingcombustion chamber 118 and exhaust outlet 116. As described above, fan114 is positioned at the discharge end of heat exchanger assembly 120or, in the alternative, at an inlet to combustion chamber 118, to inducea draft in combustion chamber 118 for moving exhaust combustion chambergases through exhaust outlet 116. The moving gases establish a pressuredrop across heat exchanger assembly 120 which forces furnace exhaustfrom combustion chamber 118 through heat exchanger assembly 120.Preferably, a blower 400 moves air across heat exchanger assembly 120for conditioning the air. The conditioned air gains heat from theexhaust combustion chamber gases before the blower 400 discharges itfrom furnace 117.

As shown in FIG. 4, combustion chamber 118 houses a burner assembly 402adjacent one end of heat exchanger assembly 120. A secondary orcondensing heat exchanger assembly, such as heat exchanger assembly120a, may be at the other end of heat exchanger assembly 120. Also, thedischarge end of heat exchanger assembly 120 may be connected to acollector box (not shown) for collecting condensate or directly toexhaust outlet 116. In operation, furnace controller 128 controls a gasvalve 404 for metering the flow of gas from fuel supply 124 to theburner assembly 402. A number of orifices in a gas manifold 406 providegas to combustion chamber 118 where it is preferably mixed with airbefore being ignited by igniter 126. In the embodiment shown in FIG. 4,the ignited air-fuel mixture is pulled through heat exchanger assembly120 in the direction shown. Draft inducer fan 114 then forces therelatively cool exhaust gases to pass through exhaust outlet 116 to theatmosphere.

In a preferred embodiment of the invention, heat exchanger assembly 120is comprised of a plurality of heat exchanger elements 408. Each heatexchanger element 408 corresponds to a burner 410 of burner assembly402. Thus, during operation of furnace 117, hot combustion chamber gasesare pulled through each of burners 410 of burner assembly 402 into acorresponding one of heat exchanger elements 408 of heat exchangerassembly 120.

Generally, the number of heat exchanger elements 408 determines thecapacity of furnace 117. As described herein, the number of heatexchanger elements 408 is also referred to as the cell size of furnace117 wherein one heat exchanger element 408 and one corresponding burner410 generally constitute a cell. Depending on the desired furnacecapacity, a number of cells may be installed side-by-side in furnace117. For example, a two-cell furnace typically has a capacity of about40,000 Btu whereas a seven-cell furnace typically has a capacity ofabout 140,000 Btu.

In general, furnaces with different capacities will have differentrestrictions to air flow. As a result, the load on draft inducer motor104 may differ from furnace to furnace and motor 104 must operate atdifferent speeds and/or torques to produce the desired pressure incombustion chamber 118. Conventional furnaces either use differentmotors for different capacity furnaces or include a device, such as achoke plate, to modify the furnace installation. By modifying thefurnace installation, a higher capacity furnace is simulated byincreasing the restriction of a lower capacity furnace. In this manner,the draft inducer operates as if it is installed in a high capacityfurnace at the expense of efficiency. Advantageously, the presentinvention automatically determines the furnace capacity as a function ofrestriction and operates motor 104 accordingly. Thus, system 100eliminates the need for a choke plate and increases efficiency yetallows a single motor to be used in a number of furnace applications.

According to a preferred embodiment of the present invention, memory 132stores information defining a relationship between motor speed, motortorque and parameters defining motor operation. Processor 150, which ispreferably part of control circuit 102, determines the speed and torqueof motor 104 and retrieves at least one parameter from memory 132 whichcorresponds to the determined motor speed and motor torque. Processor150 is responsive to the retrieved parameter for defining at least oneset of speed/torque curves, such as the curves shown in FIG. 2,corresponding to a desired pressure drop across heat exchanger assembly120. Control circuit 102 then generates the motor control signal inresponse to the defined set of speed/torque curves so that motor 104will operate as a function of the determined furnace capacity to controlthe draft induced in combustion chamber 118. In this manner, controlcircuit 102 provides adaptive control of furnace 117 based onrestriction differences due to differences in furnace capacity but alsodue to any other cause.

In addition to the information relating motor operation to the speed andtorque of motor 104, memory 132 preferably stores a table of predefinedspeed/torque values for defining the speed/torque curves under varyingfurnace conditions. In one embodiment, the retrieved parameterrepresents the number of heat exchanger elements 408 in heat exchangerassembly 120 for determining the capacity of the particular furnace 117.

In one preferred embodiment of the invention, memory 132 storesspeed/torque points defining several sets of speed/torque curves, suchas the curves shown in FIG. 2, wherein each set corresponds to aparticular furnace capacity. The speed/torque curves corresponding tothe different capacities will be generally parallel to each otherwherein the curves for a lower capacity furnace will demand less torquethan the curves for a higher capacity furnace. Processor 150 generates aset of appropriate speed/torque curves from the stored points or selectsa set of appropriate curves from several stored in memory 132 foroperating motor 104 depending on the furnace capacity.

In the alternative, the retrieved parameter defines at least one deltavalue corresponding to the number of heat exchanger elements 408.Processor 150 adapts each of the predefined speed/torque values as afunction of the delta value thereby to define a set of speed/torquecurves corresponding to the desired pressure drop across heat exchangerassembly 120 based on the determined furnace capacity. By determiningthe cell size of furnace 117, processor 150 is able to adapt a singleset of speed/torque curves stored in memory 132 for use with any numberof different furnaces having different capacities.

As described above, furnace 117 is operable in first and secondoperating states. According to the present invention, processor 150 isresponsive to the retrieved parameter for defining first and second setsof speed/torque curves corresponding to the desired pressure drop acrossheat exchanger assembly 120 for the first and second operating states,respectively. Thus, motor 104 operates in accordance with one or more ofthe first speed/torque curves when furnace 117 is in the first operatingstate and in accordance with one or more of the second speed/torquecurves when furnace 117 is in the second operating state.

Also, furnace 117 includes furnace controller 128 providing a furnaceoperating signal for causing furnace 117 to operate in either a highstage or a low stage. In a preferred embodiment, processor 150 isresponsive to the retrieved parameter for defining first and second setsof speed/torque curves corresponding to the desired pressure drop acrossheat exchanger assembly 120 for the high and low stages, respectively.Thus, motor 104 operates in accordance with one or more of the firstspeed/torque curves when furnace 117 is operating in the high stage andin accordance with one or more of the second speed/torque curves whenfurnace 117 is operating in the low stage.

FIGS. 5A and 5B illustrate the operation of system 100 in the form of apreferred flow diagram 500 beginning at step 502. Processor 150initializes its operation at step 504 by instructing control circuit 102to generate the motor control signal to set the current value so thatthe windings of motor 104 are energized to produce a first torque, suchas 15% of the maximum motor torque. Processor 150 also initializes atimer 152, preferably resident in control circuit 102, for timing afirst interval of time during which motor 104 operates to produce thefirst torque. At step 504, processor 150 also sets the first intervalto, for example, 204.8 ms.

As described above, a speed circuit, preferably resident in controlcircuit 102, provides a speed signal representative of the speed ofmotor 104 in response to the speed/torque feedback via line 110. Asmotor 104 rotates fan 114, processor 150 compares the motor speed asrepresented by the speed signal to a reference speed, such as 2700 rpm,at step 506. The timer 152 times the first interval during which motor104 produces the first torque. If the motor speed is less than thereference speed at step 506, processor 150 determines at step 508whether timer 152 has timed out the first interval. If not, processor150 repeats steps 506 and 508. On the other hand, if timer 152 times outthe first interval and the motor speed is still less than the referencespeed, processor 150 proceeds to step 510.

At step 510, processor 150 resets timer 152 and causes control circuit102 to incrementally increase the current value for increasing thetorque of motor 104. Thus, control circuit 102 is responsive to thespeed signal and timer 152 for generating the appropriate motor controlsignal to incrementally increase the motor torque above the first torqueif the speed signal indicates that the speed of motor 104 is less thanthe reference speed after the first interval of time is timed by timer152. Processor 150 causes control circuit 102 to continue toincrementally increase the motor torque at intervals of 204.8 ms whenthe speed of motor 104 is less than the reference speed.

Processor 150 alters the current ramp rate at steps 512 and 514. Ifprocessor 150 determines at step 514 that the motor torque has beenincreased in excess of a second torque, such as 40% of the maximumtorque, processor 150 resets the timed interval at step 516 to a secondinterval of time less than the first interval. For example, the secondinterval is 102.4 ms. Processor 150 causes control circuit 102 tocontinue to incrementally increase the motor torque at intervals of102.4 ms when the speed of motor 104 is less than the reference speed.If processor 150 determines at step 512 that the motor torque exceeds,for example, 60% of the maximum torque, processor 150 resets the timedinterval at step 518 to a third interval of time less than the secondinterval. The third interval is, for example, 51.2 ms. Processor 150then causes control circuit 102 to incrementally increase the motortorque every 51.2 ms as long as the motor speed remains less than thereference speed.

Processor 150 operates to ramp the current and, thus, the motor torqueat the variable rate described above with respect to steps 504, 512,514, 516 and 518 until the motor speed reaches the reference speed. Asshown in flow diagram 500, the variable ramp rate is initially slowerbecause at low speeds, relatively small changes in torque can causerelatively large changes in motor speed. After successively increasingthe current value every 204.8 ms, the ramp rate is increased byincreasing the current value every 102.8 ms as needed to achieve thereference speed and then increasing the current value every 51.2 ms asneeded to achieve the reference speed. Once processor 150 determines atstep 506 that the motor speed has reached the reference speed, processor150 proceeds to operate according to the portion of flow diagram 500shown in FIG. 5B.

At steps 520, 522, 524, 526 and 528, processor 150 preferably comparesthe determined motor torque to a plurality of torque ranges stored inmemory 132. Processor 150 determines the torque of motor 104 when themotor speed reaches the reference speed so that the parameter whichcorresponds to the determined motor torque and the reference motor speedis retrieved from memory 132. In this embodiment, the retrievedparameter corresponds to one of the torque ranges which includes thedetermined motor torque. As an example, if the motor torque needed toachieve the reference speed exceeds 27.8% at step 520, processor 150retrieves a parameter at step. 530 which corresponds to seven heatexchanger elements 408. In other words, processor 150 determines thatthe cell size of furnace 117 is seven. Further to the example, ifprocessor 150 determines at step 522 that the motor torque is between27.8% and 26.5%, it retrieves a parameter corresponding to a cell sizeof six at step 532; if processor 150 determines at step 524 that themotor torque is between 26.5% and 25.2%, it retrieves a parametercorresponding to a cell size of five at step 534; if processor 150determines at step 526 that the motor torque is between 25.2% and 23.9%,it retrieves a parameter corresponding to a cell size of four at step536; if processor 150 determines at step 528 that the motor torque isbetween 23.9% and 22%, it retrieves a parameter corresponding to a cellsize of three at step 538; and if processor 150 determines at step 528that the motor torque is less than 22%, it retrieves a parametercorresponding to a cell size of two at step 540.

In one preferred embodiment of the invention, at step 542, processor 150looks up a table of predefined speed/torque values stored in memory 132in response to the retrieved parameter. Processor 150 uses the storedvalues to define speed/torque curves under varying furnace conditions.As an example, the stored table includes a set of four points definingeach of the speed/torque curves of FIG. 2. Control circuit 102 isresponsive to the retrieved parameter for generating motor controlsignals as a function of a set of speed/torque curves developed byperforming a three-piece linear fit of the four points stored in memory132. As such, processor 150 defines a speed/torque curve correspondingto, for example, pre-fire low stage operation. Processor 150 preferablylooks up additional sets of speed/torque points to similarly defineappropriate curves for post-fire low stage operation as well as pre- andpost-fire high stage operation. In the alternative, processor 150retrieves a delta, or offset, value from memory 132 corresponding to thedetermined furnace capacity and adds or subtracts it from the definedspeed/torque curve to define additional curves for operating furnace 117in another mode.

In the alternative, processor 150 defines the speed and/or torque ofmotor 104 by retrieving a parameter defining at least one delta valuecorresponding to the number of heat exchanger elements 408. Processor150 adapts each of the applicable speed/torque values stored in memory132 as a function of the delta value thereby to define a set ofspeed/torque curves corresponding to the desired pressure drop acrossheat exchanger assembly 120 based on the determined furnace capacity.

FIGS. 6A and 6B illustrate the operation of system 100 according toanother preferred embodiment of the invention in the form of a flowdiagram 600 beginning at step 602. Processor 150 initializes itsoperation at step 604 by instructing control circuit 102 to generate themotor control signal to set the current value so that the windings ofmotor 104 are energized to produce a reference torque, such as 21% ofthe maximum motor torque. In this embodiment, processor 150 determinesthe speed of motor 104 which results when motor 104 is operating toproduce the reference torque.

Processor 150 preferably determines the motor speed a period of timeafter motor 104 begins operating and, thus, allows the motor speed tostabilize. At step 606, processor 150 delays for a period of time, suchas five seconds, after motor 104 begins rotating fan 114. Processor 150then measures the motor speed as represented by the speed signal at step608.

At steps 610, 612, 614, 616 and 618, processor 150 preferably comparesthe determined motor speed to a plurality of speed ranges stored inmemory 132. Processor 150 determines the speed of motor 104 when themotor torque is at the reference torque so that the parameter whichcorresponds to the reference motor torque and the determined motor speedis retrieved from memory 132. In this embodiment, the retrievedparameter corresponds to one of the speed ranges which includes thedetermined motor speed. As an example, if the motor speed which resultswhen motor 104 is operating to produce the reference torque exceeds 2600rpm at step 610, processor 150 retrieves a parameter at step 620 whichcorresponds to two heat exchanger elements 408. In other words,processor 150 determines that the cell size of furnace 117 is two.Further to the example, if processor 150 determines at step 612 that themotor speed is between 2600 rpm and 2420 rpm, it retrieves a parametercorresponding to a cell size of three at step 622; if processor 150determines at step 614 that the motor speed is between 2420 rpm and 2310rpm, it retrieves a parameter corresponding to a cell size of four atstep 624; if processor 150 determines at step 616 that the motor speedis between 2310 rpm and 2200 rpm, it retrieves a parameter correspondingto a cell size of five at step 626; if processor 150 determines at step618 that the motor speed is between 2200 rpm and 2100 rpm, it retrievesa parameter corresponding to a cell size of six at step 628; and ifprocessor 150 determines at step 618 that the motor speed is less than2100 rpm, it retrieves a parameter corresponding to a cell size of sevenat step 630.

In a manner similar to flow diagram 500, at step 632, processor 150looks up a table of predefined speed/torque values stored in memory 132for defining speed/torque curves, such as the curves shown in FIG. 2,under varying furnace conditions.

FIGS. 7A and 7B illustrate the operation of system 100 according to yetanother preferred embodiment of the invention in the form of a flowdiagram 700 beginning at step 702. According to the flow diagram 700,control circuit 102 ramps the current value to increase motor torque ina manner similar to flow diagram 500. In this embodiment, both torqueand speed are determined when pressure switch 146 activates. Based onthe predefined information stored in memory 132, processor 150 thendetermines the cell size of furnace 117.

Processor 150 initializes its operation at step 704 by instructingcontrol circuit 102 to generate the motor control signal for energizingthe windings of motor 104 to produce a first torque, such as 15% of themaximum motor torque. Processor 150 also initializes timer 152 fortiming the first interval of time during which motor 104 operates toproduce the first torque. At step 704, processor 150 also sets the firstinterval to, for example, 204.8 ms.

As described above, pressure switch 146 provides a pressure signalrepresentative of a predetermined pressure across heat exchangerassembly 120. The predetermined pressure may be representative of aminimum pressure or, in the alternative, a pressure set point forcalibrating system 100. As motor 104 rotates fan 114, processor 150determines at step 706 when pressure switch 146 indicates that thepressure drop has reached the predetermined pressure. Timer 152 timesthe first interval of time during which motor 104 produces the firsttorque. If the pressure drop across heat exchanger assembly 120 is lessthan the predetermined pressure at step 706, processor 150 determines atstep 708 whether timer 152 has timed out the first interval. If not,processor 150 repeats steps 706 and 708. On the other hand, if timer 152times out the first interval and the pressure drop is still less thanthe predetermined pressure, processor 150 proceeds to step 710.

At step 710, processor 150 resets timer 152 and causes control circuit102 to incrementally increase the current value for increasing thetorque of motor 104. Thus, control circuit 102 is responsive to thepressure signal and timer 152 for generating the appropriate motorcontrol signal to incrementally increase the motor torque above thefirst torque if the pressure signal indicates that the pressure dropacross heat exchanger assembly 120 is less than the predeterminedpressure after the first interval of time is timed by timer 152.Processor 150 causes control circuit 102 to continue to incrementallyincrease the motor torque at intervals of 204.8 ms when the pressuredrop is less than the predetermined pressure.

Processor 150 alters the current ramp rate at steps 712 and 714. Ifprocessor 150 determines at step 714 that the motor torque has beenincreased in excess of a second torque, such as 40% of the maximumtorque, processor 150 resets the timed interval at step 716 to a secondinterval of time less than the first interval. For example, the secondinterval is 102.4 ms. Processor 150 causes control circuit 102 tocontinue to incrementally increase the motor torque at intervals of102.4 ms when the pressure drop is less than the predetermined pressure.If processor 150 determines at step 712 that the motor torque exceeds,for example, 60% of the maximum torque, processor 150 resets the timedinterval at step 718 to a third interval of time less than the secondinterval. The third interval is, for example, 51.2 ms. Processor 150then causes control circuit 102 to incrementally increase the motortorque every 51.2 ms as long as the pressure drop remains less than thepredetermined pressure.

Processor 150 operates to ramp the current and, thus, the motor torqueat the variable rate described above with respect to steps 704, 712,714, 716 and 718 until pressure switch 146 activates. After successivelyincreasing the current value every 204.8 ms, the ramp rate is increasedby increasing the current value every 102.8 ms as needed to achieve thepredetermined pressure and then increasing the current value every 51.2ms as needed to achieve the predetermined pressure. Once pressure switch146 activates at step 706, processor 150 proceeds to operate accordingto the portion of flow diagram 700 shown in FIG. 7B. In this embodiment,pressure switch 146 activates when the pressure drop across heatexchanger assembly 120 is 0.25 inches of water for an 80%, two-stagefurnace, for example.

At step 720, processor 150 determines the motor torque needed toactivate pressure switch 146 and, based on the determined motor torque,calculates a torque value with respect to a reference speed. In otherwords, processor 150 normalizes the determined motor torque. In thisembodiment, the stored information includes a plurality of torque rangeswherein the retrieved parameter corresponds to one of the torque rangeswhich includes the normalized motor torque.

As an example, processor 150 determines the motor torque when pressureswitch 146 activates and normalizes it to a reference speed of 1950 rpm(represented by SPEED_(REF) =100, where SPEED is an eight-bit numberbetween 0 and 255 corresponding to the speed of motor 104). By doing so,processor 150 accommodates for the fact that the motor speed will varywhen the predetermined pressure is reached depending on the restrictionof furnace 117 and that the determined motor torques for differentcapacity furnaces may overlap if not calculated with respect to areference speed. The reference speed is preferably selected as theminimum desired speed for calculating the normalized torque value.Processor 150 normalizes the motor torque by subtracting the referencespeed, 1950 rpm, from the determined speed as represented by the speedsignal when pressure switch 146 indicates that the pressure drop acrossheat exchanger assembly 120 is the predetermined pressure. Thedifference (i.e., SPEED-SPEED_(REF)) is multiplied by a slope, m, andthen subtracted from the determined motor torque. Thus, processor 150solves for the normalized motor torque by the equation: TORQUE₁₉₅₀=TORQUE-m(SPEED-100). in a preferred embodiment, the normalized valuesfor a two-cell furnace and for a seven-cell furnace diverge when m=0.5(see Table 1, below).

                  TABLE 1                                                         ______________________________________                                               TWO-CELL      SEVEN-CELL                                               SPEED (RPM)                                                                            TORQUE   TORQUE.sub.1950                                                                          TORQUE TORQUE.sub.1950                           ______________________________________                                        1950     50       50         75     75                                        2200     54       48         83     77                                        2500     60       46         91     77                                        3000     70       43         106    79                                        3500     83       43.5       123    83.5                                      4000     97       44.5       145    92.5                                      ______________________________________                                    

Referring again to FIG. 7B, at steps 722, 724, 726, 728 and 730,processor 150 preferably compares the normalized motor torque to aplurality of torque ranges stored in memory 132. It is to be understoodthat the values of normalized torque will vary if they are calculatedwith respect to a different reference speed or if the slope m differs.As an example, if the normalized motor torque exceeds 14.6% at step 722,processor 150 retrieves a parameter at step 732 which corresponds toseven heat exchanger elements 408. In other words, processor 150determines that the cell size of furnace 117 is seven. Further to theexample, if processor 150 determines at step 724 that the normalizedmotor torque is between 14.6% and 13.7%, it retrieves a parametercorresponding to a cell size of six at step 734; if processor 150determines at step 726 that the normalized motor torque is between 13.7%and 12.9%, it retrieves a parameter corresponding to a cell size of fiveat step 736; if processor 150 determines at step 728 that the normalizedmotor torque is between 12.9% and 11.9%, it retrieves a parametercorresponding to a cell size of four at step 738; if processor 150determines at step 730 that the normalized motor torque is between 11.9%and 11.2%, it retrieves a parameter corresponding to a cell size ofthree at step 740; and if processor 150 determines at step 730 that thenormalized motor torque is less than 11.2%, it retrieves a parametercorresponding to a cell size of two at step 742.

In a manner similar to flow diagram 500 and 600, at step 744, processor150 looks up a table of predefined speed/torque values stored in memory132 for defining speed/torque curves, such as the curves shown in FIG.2, under varying furnace conditions. In the alternative, processor 150fits a straight line through the speed and torque point at whichpressure switch 146 activates for approximating the operating region ofa speed/torque curve. Processor 150 then retrieves at least one delta,or offset, value for adjusting the straight line approximation to definespeed/torque curves for pre-fire (high and low stages) and post-fire(high and low stages) operation. Further, it is to be understood thatfurnace 117 may include more than one pressure switch for providingadditional calibration points for processor 150 to define thespeed/torque curves.

FIGS. 8A and 8B illustrate the operation of system 100 according to yetanother preferred embodiment of the invention in the form of a flowdiagram 800 beginning at step 802. According to the flow diagram 800,control circuit 102 initially ramps the current value to increase motortorque in a manner similar to flow diagram 500 and 700. In thisembodiment, both torque and speed are determined when pressure switch146 activates.

Processor 150 initializes its operation at step 804 by instructingcontrol circuit 102 to generate the motor control signal for energizingthe windings of motor 104 to produce a first torque, such as 15% of themaximum motor torque. Processor 150 also initializes timer 152 fortiming the first interval of time during which motor 104 operates toproduce the first torque. At step 804, processor 150 also sets the firstinterval to, for example, 512 ms. As motor 104 rotates fan 114,processor 150 determines at step 806 when pressure switch 146 indicatesthat the pressure drop across heat exchanger assembly 120 has reachedthe predetermined pressure. Timer 152 times the first interval of timeduring which motor 104 produces the first torque. If the pressure dropacross heat exchanger assembly 120 is less than the predeterminedpressure at step 806, processor 150 determines at step 808 whether timer152 has timed out the first interval. If not, processor 150 repeatssteps 806 and 808. On the other hand, if timer 152 times out the firstinterval and the pressure drop is still less than the predeterminedpressure, processor 150 proceeds to step 810.

At step 810, processor 150 resets timer 152 and causes control circuit102 to incrementally increase the current value for increasing thetorque of motor 104. Thus, control circuit 102 is responsive to thepressure signal and timer 152 for generating the appropriate motorcontrol signal to incrementally increase the motor torque above thefirst torque if the pressure signal indicates that the pressure dropacross heat exchanger assembly 120 is less than the predeterminedpressure after the first interval of time is timed by timer 152.Processor 150 causes control circuit 102 to continue to incrementallyincrease the motor torque every 512 ms when the pressure drop is lessthan the predetermined pressure. Once pressure switch 146 activates atstep 806, processor 150 proceeds to operate according to the portion offlow diagram 800 shown in FIG. 8B. In this embodiment, pressure switch146 activates when the pressure drop across heat exchanger assembly 120is 0.25 inches of water for an 80%, two-stage furnace, for example.

At step 812, processor 150 determines the motor torque needed toactivate pressure switch 146 and, based on the determined motor torque,calculates a torque value with respect to a reference speed in a mannersimilar to step 720 of FIG. 7B. As an example, processor 150 determinesthe motor torque when pressure switch 146 activates and normalizes it toa reference speed of 1175 rpm (represented by SPEED_(REF) =60, whereSPEED is an eight-bit number between 0 and 255 corresponding to thespeed of motor 104). Processor 150 normalizes the motor torque bysubtracting the reference speed, 1175 rpm, from the determined speed asrepresented by the speed signal when pressure switch 146 indicates thatthe pressure drop across heat exchanger assembly 120 is thepredetermined pressure. The difference (i.e., SPEED-SPEED_(REF)) ismultiplied by a slope, m, and then added to the determined motor torque.Thus, processor 150 solves for the normalized motor torque by theequation: TORQUE₁₁₇₅ =TORQUE+m(SPEED-60). In a preferred embodiment, theslope is a constant value corresponding to each of the four operatingconditions of a two-stage furnace, i.e., pre-fire (high and low stages)and post-fire (high and low stages), divided by 256.

In a manner similar to flow diagram 500, 600, and 700, at step 814,processor 150 looks up a table of predefined speed/torque values storedin memory 132 for defining speed/torque curves, such as the curves shownin FIG. 2, under varying furnace conditions. In the embodiment of FIGS.8A and 8B, processor 150 preferably defines a calibration point as afunction of the speed/torque point corresponding to the normalizedtorque value and the reference speed. Preferably, processor 150 fits astraight line through the defined calibration point for approximatingthe operating region of a speed/torque curve. According to theinvention, the calibration point is defined by a delta value stored inmemory 132 corresponding to each of the four operating conditions of atwo-stage furnace, i.e., pre-fire (high and low stages) and post-fire(high and low stages). In other words, each of the speed/torque curvesdefined at step 814 includes a point offset by the retrieved delta valuefrom the speed/torque point corresponding to the normalized torque valueand the reference speed in one preferred embodiment.

Further, it is to be understood that furnace 117 may include more thanone pressure switch for providing additional calibration points forprocessor 150 to define the speed/torque curves. According to analternative embodiment of the invention, pressure switch 146 is embodiedas a low pressure switch for determining when the pressure drop acrossheat exchanger assembly 120 corresponds to a low stage minimum pressureand as a high pressure switch for determining when the pressure dropacross heat exchanger assembly 120 corresponds to a high stage minimumpressure. Thus, processor 150 preferably defines a low stage calibrationpoint as a function of the normalized motor torque value at thereference speed when the low pressure switch activates and defines ahigh stage calibration point as a function of the normalized motortorque value at the reference speed when the high pressure switchactivates. In this instance, processor 150 preferably fits a straightline through the defined low stage and high stage calibration points forapproximating the operating region of a speed/torque curve. The lowstage calibration point is defined by a delta value stored in memory 132corresponding to each of the low stage (pre-fire and post-fire)operating conditions and the high stage calibration point is defined bya delta value stored in memory 132 corresponding to each of the highstage (pre-fire and post-fire) operating conditions. For the twopressure switch embodiment, the interval timed by timer 152 preferablyincreases from 512 ms to 1024 ms upon closure of the low pressure switchto decrease the acceleration of motor 104.

FIGS. 9A-9F illustrate exemplary speed vs. torque curves defining theoperating conditions of motor 104 according to flow diagram 800 of FIGS.8A and 8B. The exemplary speed/torque curves are illustrated as smoothlines for clarity. In particular, FIGS. 9A, 9B, 9C and 9D showspeed/torque curves defined at step 814 in a single pressure switchembodiment of the invention; FIG. 9E shows speed/torque curves definedat step 814 in a high and low pressure switch embodiment of theinvention; and FIG. 9F shows speed/torque curves defined at step 814using a four-point, three-piece linear fit rather than the straight linefit of FIGS. 9A-9D. It is to be understood that the curves of FIGS.9A-9F, as well as the curves of FIG. 2, are merely exemplary and mayvary for applications other than those specifically disclosed herein.Further, processor 150 is capable of defining speed/torque curves suchas those shown in FIGS. 9A-9F, as well as those shown in FIG. 2,according to flow diagrams 500, 600 and/or 700.

In each of FIGS. 9A-9F, the speed/torque point corresponding to thenormalized torque value and the reference speed for the particular setof curves is shown at reference character P. As shown, each of thespeed/torque curves are offset from the speed/torque point P by theretrieved delta value corresponding to pre-fire (high and low stages)and post-fire (high and low stages) operation, respectively. Further,the exemplary curves of FIGS. 9A-9F include portions for limiting themaximum and minimum duty cycle as a function of the maximum and minimumoperating points for low and high stage, cold and fired operation. Ingeneral, FIGS. 9A-9F illustrate curves for various points P and/orvarious maximum/minimum operating points for either cold or firedoperation.

In yet another preferred embodiment of the present invention, memory 132stores parameters representative of error corrections to compensate forthe errors associated with pressure switch tolerances as well as motortolerances. For example, if pressure switch 146 activates in response toa pressure drop across heat exchanger assembly 120 of 0.25±0.05 inchesof water, processor 150 adjusts its calculations based on the actualpressure at which pressure switch 146 activates as measured during finaltesting of furnace 117.

Although FIGS. 5A-5B, 6A-6B, 7A-7B and 8A-8B are directed to operationof processor 150 to distinguish between cell sizes ranging from two toseven, it is to be understood that the present invention is alsoapplicable to determine the capacity of furnaces of any size.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions and methodswithout departing from the scope of the invention, it is intended thatall matter contained in the above description and shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A draft inducer apparatus for use with a furnacethat includes a combustion chamber and an exhaust outlet for ventingexhaust combustion chamber gases from the furnace and that also includesat least one heat exchanger assembly for extracting heat from theexhaust combustion chamber gases and transferring heat to conditionedair, and for use with a fan for inducing a draft in the combustionchamber which causes a pressure drop across the heat exchanger assemblyfor moving the exhaust combustion chamber gases through the exhaustoutlet, said heat exchanger assembly having a plurality of heatexchanger elements which determine furnace capacity wherein the pressuredrop across the heat exchanger assembly is affected by the furnacecapacity, said apparatus comprising:a motor including a shaft fordriving the fan in response to a motor control signal; a memory forstoring information defining a relationship between motor speed, motortorque and parameters defining motor operation; a processor fordetermining the speed and torque of the motor and for retrieving fromthe memory a retrieved parameter corresponding to the determined motorspeed and motor torque, said processor being responsive to the retrievedparameter for defining at least one set of speed/torque curvescorresponding to a desired pressure drop across the heat exchangerassembly; and a control circuit for generating the motor control signalin response to the defined set of speed/torque curves thereby to controlthe draft induced in the combustion chamber whereby the motor isoperated as a function of the determined motor speed and motor torquewhich are indicative of the furnace capacity.
 2. The apparatus of claim1 wherein the memory is part of the processor and stores a table ofpredefined speed/torque values for defining the speed/torque curvesunder varying furnace conditions.
 3. The apparatus of claim 2 whereinthe retrieved parameter represents the number of heat exchanger elementsin the heat exchanger assembly for determining the furnace capacity,wherein the retrieved parameter defines at least one delta valuecorresponding to the number of heat exchanger elements, and wherein theprocessor adapts each of the predefined speed/torque values as afunction of the delta value thereby to define a set of speed/torquecurves corresponding to the desired pressure drop across the heatexchanger assembly based on the determined furnace capacity.
 4. Theapparatus of claim 1 further comprising a speed circuit providing aspeed signal representative of the speed of the motor and wherein thecontrol circuit is responsive to the speed signal for generating themotor control signal to increase the speed of the motor when the speedsignal indicates that the motor speed is less than a reference speed andwherein the processor determines the torque of the motor when the motorspeed corresponds to the reference speed so that the parameter whichcorresponds to the determined motor torque and the reference motor speedis retrieved from the memory.
 5. The apparatus of claim 4 wherein thecontrol circuit generates the motor control signal so that the motoroperates to produce a first torque which is increased to increase thespeed of the motor when the speed signal indicates that the motor speedis less than the reference speed.
 6. The apparatus of claim 5 furthercomprising a timer for timing a first interval of time during which themotor operates to produce the first torque and wherein the controlcircuit is responsive to the timer for generating the motor controlsignal so that the motor operates to incrementally increase the motortorque above the first torque if the speed signal indicates that thespeed of the motor is less than the reference speed after the firstinterval of time is timed by the timer.
 7. The apparatus of claim 6wherein the timer further times a second interval of time less than thefirst interval of time during which the motor operates to produce asecond torque greater than the first torque and wherein the controlcircuit is responsive to the timer for generating the motor controlsignal so that the motor operates to incrementally increase the motortorque above the second torque if the speed signal indicates that thespeed of the motor is less than the reference speed after the secondinterval of time is timed by the timer.
 8. The apparatus of claim 4wherein the stored information includes a plurality of torque ranges andthe retrieved parameter corresponds to one of the torque ranges whichincludes the determined motor torque.
 9. The apparatus of claim 1wherein the control circuit generates the motor control signal so thatthe motor operates to produce a reference torque and wherein theprocessor determines the speed of the motor which results when the motoris operating to produce the reference torque so that the parameter whichcorresponds to the determined motor speed and the reference motor torqueis retrieved from the memory.
 10. The apparatus of claim 9 wherein theprocessor determines the speed of the motor a period of time after themotor begins operating to produce the reference torque.
 11. Theapparatus of claim 9 wherein the stored information includes a pluralityof speed ranges and the retrieved parameter corresponds to one of thespeed ranges which includes the determined motor speed.
 12. Theapparatus of claim 1 further comprising a pressure switch providing apressure signal representative of a predetermined pressure across theheat exchanger assembly and wherein the control circuit is responsive tothe pressure signal for generating the motor control signal to increasethe torque of the motor when the pressure signal indicates that thepressure drop across the heat exchanger assembly is less than thepredetermined pressure and wherein the processor determines the speedand torque of the motor when the pressure drop corresponds to thepredetermined pressure so that the parameter which corresponds to thedetermined motor speed and motor torque is retrieved from the memory.13. The apparatus of claim 12 wherein the control circuit generates themotor control signal so that the motor operates to produce a firsttorque which is increased to increase the torque of the motor when thepressure signal indicates that the pressure drop across the heatexchanger assembly is less than the predetermined pressure.
 14. Theapparatus of claim 13 further comprising a timer for timing a firstinterval of time during which the motor operates to produce the firsttorque and wherein the control circuit is responsive to the timer forgenerating the motor control signal so that the motor operates toincrementally increase the motor torque above the first torque if thepressure signal indicates that the pressure drop across the heatexchanger assembly is less than the predetermined pressure after thefirst interval of time is timed by the timer.
 15. The apparatus of claim14 wherein the timer further times a second interval of time less thanthe first interval of time during which the motor operates to produce asecond torque greater than the first torque and wherein the controlcircuit is responsive to the timer for generating the motor controlsignal so that the motor operates to incrementally increase the motortorque above the second torque if the pressure signal indicates that thepressure drop across the heat exchanger assembly is less than thepredetermined pressure after the second interval of time is timed by thetimer.
 16. The apparatus of claim 12 wherein the processor normalizesthe determined motor torque to a reference speed and wherein the storedinformation includes a plurality of torque ranges and the retrievedparameter corresponds to one of the torque ranges which includes thenormalized motor torque.
 17. The apparatus of claim 1 wherein thefurnace is operable in first and second operating states and theprocessor is responsive to the retrieved parameter for defining firstand second sets of speed/torque curves corresponding to the desiredpressure drop across the heat exchanger assembly for the first andsecond operating states, respectively, so that the motor operates inaccordance with one or more of the first speed/torque curves when thefurnace is in the first operating state and in accordance with one ormore of the second speed/torque curves when the furnace is in the secondoperating state.
 18. The apparatus of claim 1 wherein the furnaceincludes a furnace controller providing a furnace operating signal forcausing the furnace to operate in either a high stage or a low stage andwherein the processor is responsive to the retrieved parameter fordefining first and second sets of speed/torque curves corresponding tothe desired pressure drop across the heat exchanger assembly for thehigh and low stages, respectively, so that the motor operates inaccordance with one or more of the first speed/torque curves when thefurnace is operating in the high stage and in accordance with one ormore of the second speed/torque curves when the furnace is operating inthe low stage.
 19. A furnace system comprising:a combustion chamber; anexhaust outlet for venting exhaust combustion chamber gases from thecombustion chamber; at least one heat exchanger assembly for extractingheat from the exhaust combustion chamber gases and for transferring heatto conditioned air, said heat exchanger assembly having a plurality ofheat exchanger elements which determine furnace capacity; a fan forinducing a draft in the combustion chamber which causes a pressure dropacross the heat exchanger assembly for moving the exhaust combustionchamber gases through the exhaust outlet wherein the pressure dropacross the heat exchanger assembly is affected by the furnace capacity;a motor including a shaft for driving the fan in response to a motorcontrol signal; a memory for storing information defining a relationshipbetween motor speed, motor torque and parameters defining motoroperation; a processor for determining the speed and torque of the motorand for retrieving from the memory a retrieved parameter correspondingto the determined motor speed and motor torque, said processor beingresponsive to the retrieved parameter for defining at least one set ofspeed/torque curves corresponding to a desired pressure drop across theheat exchanger assembly; and a control circuit for generating the motorcontrol signal in response to the defined set of speed/torque curvesthereby to control the draft induced in the combustion chamber wherebythe motor is operated as a function of the determined motor speed andmotor torque which are indicative of the furnace capacity.
 20. A methodof operating a draft inducer apparatus for use with a furnace thatincludes a combustion chamber and an exhaust outlet for venting exhaustcombustion chamber gases from the furnace and that also includes atleast one heat exchanger assembly for extracting heat from the exhaustcombustion chamber gases and transferring heat to conditioned air, andfor use with a fan for inducing a draft in the combustion chamber whichcauses a pressure drop across the heat exchanger assembly for moving theexhaust combustion chamber gases through the exhaust outlet, said heatexchanger assembly having a plurality of heat exchanger elements whichdetermine furnace capacity wherein the pressure drop across the heatexchanger assembly is affected by the furnace capacity, said methodcomprising the steps of:driving the fan with a motor in response to amotor control signal; storing information in a memory defining arelationship between motor speed, motor torque and parameters definingmotor operation; determining the speed and torque of the motor;retrieving from the memory a retrieved parameter corresponding to thedetermined motor speed and motor torque; defining at least one set ofspeed/torque curves corresponding to a desired pressure drop across theheat exchanger assembly in response to the retrieved parameter; andgenerating the motor control signal in response to the defined set ofspeed/torque curves thereby to control the draft induced in thecombustion chamber whereby the motor is operated as a function of thedetermined motor speed and motor torque which are indicative of thefurnace capacity.
 21. The method of claim 20 further comprising the stepof storing a table of predefined speed/torque values in the memory fordefining the speed/torque curves under varying furnace conditions. 22.The method of claim 21 wherein the retrieved parameter represents thenumber of heat exchanger elements in the heat exchanger assembly fordetermining the furnace capacity, wherein the retrieved parameterdefines at least one delta value from the memory corresponding to thenumber of heat exchanger elements, and wherein the defining stepincludes adapting each of the predefined speed/torque values thereby todefine a set of speed/torque curves corresponding to the desiredpressure drop across the heat exchanger assembly based on the determinedfurnace capacity.
 23. The method of claim 20 further comprising thesteps of providing a speed signal representative of the speed of themotor and generating the motor control signal in response to the speedsignal to increase the speed of the motor when the speed signalindicates that the motor speed is less than a reference speed andwherein the determining step includes determining the torque of themotor when the motor speed corresponds to the reference speed so thatthe parameter which corresponds to the determined motor torque and thereference motor speed is retrieved from the memory.
 24. The method ofclaim 23 wherein the step of generating the motor control signal toincrease the motor speed includes generating the motor control signal sothat the motor operates to produce a first torque which is increased toincrease the speed of the motor when the speed signal indicates that themotor speed is less than the reference speed.
 25. The method of claim 24further comprising the step of timing a first interval of time duringwhich the motor operates to produce the first torque and wherein thestep of generating the motor control signal to produce the first torqueincludes generating the motor control signal so that the motor operatesto incrementally increase the motor torque above the first torque if thespeed signal indicates that the speed of the motor is less than thereference speed after the first interval of time is timed.
 26. Themethod of claim 25 further comprising the step of timing a secondinterval of time less than the first interval of time during which themotor operates to produce a second torque greater than the first torqueand wherein the step of generating the motor control signal toincrementally increase the motor torque includes generating the motorcontrol signal so that the motor operates to incrementally increase themotor torque above the second torque if the speed signal indicates thatthe speed of the motor is less than the reference speed after the secondinterval of time is timed.
 27. The method of claim 23 wherein thestoring step includes storing a plurality of torque ranges in the memoryand wherein the retrieved parameter corresponds to one of the pluralityof torque ranges which includes the determined motor torque.
 28. Themethod of claim 20 further comprising the step of generating the motorcontrol signal so that the motor operates to produce a reference torqueand wherein the determining step includes determining the speed of themotor which results when the motor is operating to produce the referencetorque so that the parameter which corresponds to the determined motorspeed and the reference motor torque is retrieved from the memory. 29.The method of claim 28 wherein the step of determining the speed of themotor includes determining the motor speed a period of time after themotor begins operating to produce the reference torque.
 30. The methodof claim 28 wherein the storing step includes storing a plurality ofspeed ranges in the memory and wherein the retrieved parametercorresponds to one of the plurality of speed ranges which includes thedetermined motor speed.
 31. The method of claim 20 further comprisingthe steps of providing a pressure signal representative of apredetermined pressure across the heat exchanger assembly and generatingthe motor control signal in response to the pressure signal to increasethe torque of the motor when the pressure signal indicates that thepressure drop across the heat exchanger assembly is less than thepredetermined pressure and wherein the determining step includesdetermining the speed and torque of the motor when the pressure dropcorresponds to the predetermined pressure so that the parameter whichcorresponds to the determined motor speed and motor torque is retrievedfrom the memory.
 32. The method of claim 31 wherein the step ofgenerating the motor control signal to increase the motor torqueincludes generating the motor control signal so that the motor operatesto produce a first torque which is increased to increase the torque ofthe motor when the pressure signal indicates that the pressure dropacross the heat exchanger assembly is less than the predeterminedpressure.
 33. The method of claim 32 further comprising the step oftiming a first interval of time during which the motor operates toproduce the first torque and wherein the step of generating the motorcontrol signal to produce the first torque includes generating the motorcontrol signal so that the motor operates to incrementally increase themotor torque above the first torque if the pressure signal indicatesthat the pressure drop across the heat exchanger assembly is less thanthe predetermined pressure after the first interval of time is timed.34. The method of claim 33 further comprising the step of timing asecond interval of time less than the first interval of time duringwhich the motor operates to produce a second torque greater than thefirst torque and wherein the step of generating the motor control signalto incrementally increase the motor torque includes generating the motorcontrol signal so that the motor operates to incrementally increase themotor torque above the second torque if the pressure signal indicatesthat the pressure drop across the heat exchanger assembly is less thanthe predetermined pressure after the second interval of time is timed.35. The method of claim 31 further comprising the step of normalizingthe determined motor torque to a reference speed and wherein the storingstep includes storing a plurality of torque ranges in the memory andwherein the retrieved parameter corresponds to one of the plurality oftorque ranges which includes the normalized motor torque.
 36. The methodof claim 20 wherein the furnace is operable in first and secondoperating states and wherein the defining step includes defining inresponse to the retrieved parameter first and second sets ofspeed/torque curves corresponding to the desired pressure drop acrossthe heat exchanger assembly for the first and second operating states,respectively, so that the motor operates in accordance with one or moreof the first speed/torque curves when the furnace is in the firstoperating state and in accordance with one or more of the secondspeed/torque curves when the furnace is in the second operating state.37. The method of claim 20 wherein the furnace includes a furnacecontroller providing a furnace operating signal for causing the furnaceto operate in either a high stage or a low stage and wherein thedefining step includes defining in response to the retrieved parameterfirst and second sets of speed/torque curves corresponding to thedesired pressure drop across the heat exchanger assembly for the highand low stages, respectively, so that the motor operates in accordancewith one or more of the first speed/torque curves when the furnace isoperating in the high stage and in accordance with one or more of thesecond speed/torque curves when the furnace is operating in the lowstage.
 38. A method of operating a furnace system that includes acombustion chamber and an exhaust outlet for venting exhaust combustionchamber gases from the combustion chamber and that also includes atleast one heat exchanger assembly for extracting heat from the exhaustcombustion chamber gases and for transferring heat to conditioned airand a fan for inducing a draft in the combustion chamber which causes apressure drop across the heat exchanger assembly for moving the exhaustcombustion chamber gases through the exhaust outlet, said heat exchangerassembly having a plurality of heat exchanger elements which determinefurnace capacity wherein the pressure drop across the heat exchangerassembly is affected by the furnace capacity, said method comprising thesteps of:driving the fan with a motor in response to a motor controlsignal; storing information in a memory defining a relationship betweenmotor speed, motor torque and parameters defining motor operation;determining the speed and torque of the motor; retrieving from thememory a retrieved parameter corresponding to the determined motor speedand motor torque; defining at least one set of speed/torque curvescorresponding to a desired pressure drop across the heat exchangerassembly in response to the retrieved parameter; and generating themotor control signal in response to the defined set of speed/torquecurves thereby to control the draft induced in the combustion chamberwhereby the motor is operated as a function of the determined motorspeed and motor torque which are indicative of the furnace capacity. 39.A draft inducer apparatus for use with a furnace that includes acombustion chamber and an exhaust outlet for venting exhaust combustionchamber gases from the furnace and that also includes at least one heatexchanger assembly for extracting heat from the exhaust combustionchamber gases and transferring heat to conditioned air, and for use witha fan for inducing a draft in the combustion chamber which causes apressure drop across the heat exchanger assembly for moving the exhaustcombustion chamber gases through the exhaust outlet, said heat exchangerassembly having a plurality of heat exchanger elements which determinefurnace capacity wherein the pressure drop across the heat exchangerassembly is affected by the furnace capacity, said apparatuscomprising:a motor including a shaft for driving the fan in response toa motor control signal; a memory for storing information defining arelationship between motor speed, motor torque and a number of heatexchanger elements; a speed circuit providing a speed signalrepresentative of the speed of the motor; a control circuit responsiveto the speed signal for generating the motor control signal to increasethe speed of the motor when the speed signal indicates that the motorspeed is less than a reference speed; a processor for determining thetorque of the motor when the motor speed corresponds to the referencespeed and for retrieving from the memory the number of heat exchangerelements corresponding to the determined motor torque and the referencemotor speed, said processor being responsive to the retrieved number ofheat exchanger elements for defining at least one set of speed/torquecurves corresponding to a desired pressure drop across the heatexchanger assembly, said control circuit generating the motor controlsignal in response to the defined set of speed/torque curves thereby tocontrol the draft induced in the combustion chamber whereby the motor isoperated as a function of the determined motor torque and the referencemotor speed which are indicative of the furnace capacity.
 40. A draftinducer apparatus for use with a furnace that includes a combustionchamber and an exhaust outlet for venting exhaust combustion chambergases from the furnace and that also includes at least one heatexchanger assembly for extracting heat from the exhaust combustionchamber gases and transferring heat to conditioned air, and for use witha fan for inducing a draft in the combustion chamber which causes apressure drop across the heat exchanger assembly for moving the exhaustcombustion chamber gases through the exhaust outlet, said heat exchangerassembly having a plurality of heat exchanger elements which determinefurnace capacity wherein the pressure drop across the heat exchangerassembly is affected by the furnace capacity, said apparatuscomprising:a motor including a shaft for driving the fan in response toa motor control signal; a memory for storing information defining arelationship between motor speed, motor torque and a number of heatexchanger elements; a control circuit for generating the motor controlsignal so that the motor operates to produce a reference torque; aprocessor for determining the speed of the motor which results when themotor is operating to produce the reference torque and for retrievingfrom the memory the number of heat exchanger elements corresponding tothe determined motor speed and the reference motor torque, saidprocessor being responsive to the retrieved number of heat exchangerelements for defining at least one set of speed/torque curvescorresponding to a desired pressure drop across the heat exchangerassembly, said control circuit generating the motor control signal inresponse to the defined set of speed/torque curves thereby to controlthe draft induced in the combustion chamber whereby the motor isoperated as a function of the determined motor speed and reference motortorque which are indicative of the furnace capacity.
 41. A draft inducerapparatus for use with a furnace that includes a combustion chamber andan exhaust outlet for venting exhaust combustion chamber gases from thefurnace and that also includes at least one heat exchanger assembly forextracting heat from the exhaust combustion chamber gases andtransferring heat to conditioned air, and for use with a fan forinducing a draft in the combustion chamber which causes a pressure dropacross the heat exchanger assembly for moving the exhaust combustionchamber gases through the exhaust outlet, said heat exchanger assemblyhaving a plurality of heat exchanger elements which determine furnacecapacity wherein the pressure drop across the heat exchanger assembly isaffected by the furnace capacity, said apparatus comprising:a motorincluding a shaft for driving the fan in response to a motor controlsignal; a memory for storing information defining a relationship betweenmotor speed, motor torque and a number of heat exchanger elements; apressure switch providing a pressure signal representative of a pressureacross the heat exchanger assembly; a control circuit responsive to thepressure signal for generating the motor control signal to increase thetorque of the motor when the pressure signal indicates that the pressuredrop across the heat exchanger assembly is less than the pressure; aprocessor for determining the speed and torque of the motor when thepressure drop corresponds to the pressure and for retrieving from thememory the number of heat exchanger elements corresponding to thedetermined motor speed and motor torque, said processor being responsiveto the retrieved number of heat exchanger elements for defining at leastone set of speed/torque curves corresponding to a desired pressure dropacross the heat exchanger assembly, said control circuit generating themotor control signal in response to the defined set of speed/torquecurves thereby to control the draft induced in the combustion chamberwhereby the motor is operated as a function of the determined motorspeed and motor torque which are indicative of the furnace capacity.